Considerable attention has been paid to providing improved corrosion protection to metallic surfaces. One way of providing this corrosion protection is by electrodepositing a zinc coating on the surface. For decades, electroplated zinc has been used by the automotive industry to provide an economical, highly corrosion-resitant coating. However, with today's unprecedented demands for higher quality and extended warrantees, both the automotive manufacturers and their suppliers have had to develop new coatings. The best overall performance is being demonstrated by zinc-cobalt and zinc-nickel alloy platings. These alloys are being introduced as replacements for conventional zinc electroplates in automotive as well as other applications requiring extended corrosion-resistance. The term "alloy", as used in this specification and claims is defined as a mixture of two or more metallic elements which may be microscopically homogeneous or microscopically heterogeneous.
The improvement of zinc-nickel alloys has been demonstrated by superior salt spray performance when comparing zinc-nickel to zinc electrodeposits. The amount of nickel in the zinc-nickel electrodeposit that is useful for improved corrosion protection has been found to be from about 4% nickel to about 18% nickel with an optimum level of about 10% to 12%.
Typically, zinc-nickel alloy plating baths have been based on inorganic zinc and nickel salts such as zinc sulfate, zinc chloride, nickel sulfate or nickel chloride and contain various additives to improve the brightness and the grain structure of the deposit and provide control of the zinc to nickel ratio. These baths tend to give a dull to semi-bright electrodeposit and have a tendency to give a brittle deposit. There is a commercial advantage to be able to electrodeposit a mirror-bright deposit, similar to that achieved by nickel or chrome electrodeposits and to increase the ductility of the electrodeposit.